twisted.protocols.loopback.LoopbackRelay has no unit tests

Description

Only twisted.conch.test uses this class, but that's kinda unfortunate considering there's no evidence that the class works at all.

Perhaps we should port the Conch tests to one of the other helpers then deprecate LoopbackRelay (then later remove it). Alternatively, someone could actually write unit tests for it, but that seems like a waste, given that it's somewhat redundant.